Many machines including electrical systems, such as aircraft, utilize a power distribution system to distribute AC power to one or more electronic systems within the machine. At least some of the electronic systems act as an inductive load. An inductive load creates a phase lag on current and stores energy in the inductive flux at the load. When the inductive load is switched off, any energy stored in the inductive flux at the time of the switch off must be dissipated as the flux collapses. If the inductive load is switched off at exactly the point where the AC current reverses polarity (referred to as a zero crossing), then minimal, if any, energy is stored in the inductive load resulting in a low amount of required switching dissipation. If the inductive load is not switched at the zero crossing, then significant energy can be stored within the inductive load and must be dissipated.
In systems using mechanical switches and toggles, the majority of the energy is dissipated in arcing that occurs at the switch or toggle when the physical disconnect occurs. Modern systems, however, frequently utilize solid state power controllers in place of the previous mechanical switches and toggles. When a solid state power controller is used to open (switch off) an inductive load, absent other protections, the energy is dissipated within the semiconductor device, which may be one or more metal oxide semiconductor field effect transistors (MOSFETs) contained within the solid state power controller switch. Dissipation of the stored energy within the MOSFETs can cause junction temperatures within the MOSFETs to rise rapidly and exceed the temperature design limit of the MOSFET, placing stresses on, and potentially damaging, the MOSFET.
Existing products that utilize semiconductor switching normally turn the load at approximately the current zero crossing so that the semiconductors do not have to absorb large stored energies. However, existing control methods for tracking the zero crossing result in some error in the optimum time to switch the devices off and there is residual energy left in the inductive load that still must be dissipated by the semiconductor switches. Different circuit configurations result in different error terms and different energy and heating effects on the semiconductor switches.